1. Field of the Invention
The present invention relates to the pixel circuit and a driving method of an active matrix display comprising light-emitting devices which emits light by conducting a driving current through a light emitting thin film such as an organic semiconductor thin film, and thin film transistors for controlling the light emitting operation of the respective light emitting devices. A preferred embodiment of the present invention applies to light emitting devices formed with organic material, the organic light emitting diode (OLED). More specifically, the present invention provides a method and structure to address and deliver the driving power to a pixel using multi-functional access lines, thereby simplifying the array structure of a light emitting device display and the fabrication process thereof, and increasing the fill factor of light emitting area.
2. Description of the Prior Art
Organic light emitting diode displays have attracted significant interests in commercial application in recent years. Its excellent form factor, fast response time, lighter weight, low operating voltage, and prints-like image quality make it the ideal display devices for a wide range of application from cell phone screen to large screen TV. Passive OLED displays, with relatively low resolution, have already been integrated into commercial cell phone products. Next generation devices with higher resolution and higher performance using active matrix OLEDs are being developed. Initial introduction of active matrix OLED displays have been seen in such products as digital camera and small video devices. Demonstration of OLED displays in large screen sizes further propels the development of a commercially viable active matrix OLED technology. The major challenges in achieving such a commercialization include (1) improving the material and device operating life, and (2) reducing device variation across the display area. Several methods have been suggested to address the second issue by including more active switching devices in a pixel, or by switching of supply lines externally. A common theme of these solutions is an increase of device complexity. The present invention addresses the complexity issue by structuring the pixel so that a conventional scanning electrode is configured as a current supply electrode to the light emitting device in part of a cycle to deliver full drive power, without adding to the circuit any additional switching electrode or signals.
Examples of using organic material to form an LED are found in U.S. Pat. Nos. 5,482,896, 5,408,109, and 5,663,573, and examples of using organic light emitting diode to form active matrix display devices are found in U.S. Pat. Nos. 5,684,365 and 6,157,356, all of which are hereby incorporated by reference.
An active matrix OLED display (FIG. 1) is typically structured with “SELECT” electrodes for row select, “DATA” electrodes for setting the pixel state, power electrodes VDD to drive the pixels, and a reference voltage. A basic pixel in an active matrix display also contains at least one transistor for data control, and at least a memory element to hold the data sufficiently long so a pixel remains stable in a frame. A circuit diagram for a basic pixel 100 in an active matrix OLED display is depicted in FIG. 2 in further detail. An active matrix with pixel circuit structured similar to FIG. 2 allows data to be written and retained in a storage capacitor 204 according to the data signal delivered in an address cycle, while the power supply VDD continuously drives OLED 205 through an n-channel transistor 201, according to the data setting in capacitor 204. The selection of pixels to receive data information is controlled by an n-channel transistor 203 that is controlled by the voltage on a select electrode connected to the gate of transistor 203. An active matrix driving scheme allows the drive transistor 201 remain in a data state for an extended period of time after the address cycle, the peak current required for achieving a brightness level is reduced accordingly compared to a passive matrix. Its peak driving current does not scale with the resolution, making it suitable for high resolution applications. Stability of the display is also improved appreciably.
FIG. 2b illustrates a similar construction of an active matrix light emitting device display with n-channel drive transistor 201b, n-channel data control transistor 203b, capacitor 204b, and light emitting diode 205b. 
Noticeable in both FIGS. 2 and 2b, the pixel circuits are structured as common-cathode where the cathode of the light emitting diode is connected to a common voltage line shared by other pixels. Where in FIG. 2 the data signal is written into the capacitor referencing to a constant VDD, the actual drive control voltage, gate-to-source voltage (VGS), that determines the current in the drive transistor is affected by the voltage across the light emitting diode according to, since the source terminal of the drive transistor floats on the light emitting diode. Where in FIG. 2b, the data signal is written into the capacitor being directly affected by voltage across the light emitting diode according to VGS=VDATA−VLED. In another word, in both of these examples, the gate voltage can not be directly referenced to the source of the transistor. The drive voltage is unavoidably interfered by the voltage across the light emitting diode. This illustrates a commonly know compromise between common-cathode structure and source-referencing scheme in a light emitting diode display pixel.
As illustrate in the above example, the electrical current for producing light output flows through at least a control element that regulates the current. In a conventional light emitting device display, these control elements are fabricated on a thin film of amorphous silicon on glass. Power consumed in such control elements are converted to heat rather than yielding any light. To reduce such power consumption, polycrystalline silicon is preferred over amorphous silicon for its better mobility. More elaborated methods employing self-regulated multiple-stage conversions suitable for pixel circuit using polysilicon base material may be found in U.S. Pat. Nos. 6,501,466 and 6,580,408. These methods provide a current drive scheme while largely eliminated the impact from material and transistor non-uniformity typically associated with thin film polysilicon on glass. In these methods, typically a minimum of four transistors are required to achieve such self-regulated, multi-stage conversion to achieve a pixel-independent current drive for the display. An example of such methods is illustrated in FIG. 3. where four transistors 301, 302, 303, and 307, and 3 access lines, DATA, SELECT, and VDD, are used for each pixel with a storage capacitor 304 and an OLED 305.
The present invention provides a multi-functional scan-power electrode for pixel access that carries the conventional pixel select function and power delivery function on the same bus line, thereby allowing a reduction in display complexity. The pixel structure so constructed comprises a direct current path from scan-power electrode to the light emitting element, the turning-on and off of which are fully controlled by the voltage applied on said scan-power electrodes.